1. Field of the Invention
The present invention relates to controlling transmission of sets of data and to transmitting sets of data. In particular the present invention relates to controlling transmission when at least a part of a set of data may be transmitted for a number of times.
2. Description of the Related Art
A communication system can be seen as a facility that enables communication between two or more entities such as user equipment and/or other nodes associated with the system. The communication may comprise, for example, communication of voice, data, multimedia and so on. The communication system may be circuit switched or packet switched. The communication system may be configured to provide wireless communication.
In a communication system, there is typically need to adjust transmission parameters to changing channel conditions. Link adaptation refers to changing transmission parameters of a communication channel to compensate for the variations in the channel conditions. Transmission parameters refer here to, for example, transmission power and transmission rate. In the following, transmission rate control is discussed in more detail.
In a communication system, data transmission may be defined as combinations of channel coding methods, modulation techniques, and, in a spread spectrum system, spreading spectrum techniques. Such a combination defines a specific transmission rate for user data. A combination of a channel coding method and a modulation technique is often referred to as a modulation and coding scheme (MCS).
One example of a known link adaptation technique is Adaptive Modulation and Coding Scheme (AMCS). In AMCS, a modulation and coding scheme (MCS) is selected for transmission based on estimated receiver signal quality. The basic principle is to use higher modulation levels and higher channel coding rates, when the channel condition is good, and lower modulation levels and lower channel coding rates, when the channel condition is not so good.
Typically the modulation and coding schemes are arranged to an order in accordance with the respective transmission rates, and the modulation and coding schemes are called MCS levels. The selection from a set of modulation and coding schemes is made using thresholds. Thresholds are defined in terms of channel quality, for example, as signal-to-interference ratio (SIR) values. Given an estimated channel quality, the MCS having a threshold value smaller than the given estimated channel quality and providing the largest transmission rate is typically selected in AMCS.
It has been noted that correct switching points between adjacent MCS schemes are dependent, for example, on the channel Doppler experienced by the receiver and on the multipath propagation. Fixed threshold values are therefore not sufficient for efficient link adaptation using AMCS. There have been a number of proposals for adjusting the thresholds.
In “Selection of MCS levels in HSDPA”, a TSG-RAN working group document TSG R1-01-0579 by NEC and Telecom Domus, link adaptation in a High Speed Downlink Packet Access (HSDPA) in a Wideband Code Division Multiple Access (WCDMA) system is discussed. It is proposed to adjust the threshold values for the MCS levels according to cyclic redundancy check (CRC) results of a data received at the mobile station. The mobile station sends acknowledgement messages accordingly to the CRC results to a base station. The threshold values are updated based on ACK and NAK messages received at the base station. In “Adaptive Control of Link Adaptation for High Speed Downlink Packet Access (HSDPA) in W-CDMA”, by M. Nakamura, Y. Awad and S. Vadgama, in Proceedings WPMD, pp. 382-386, September 2002, the above mentioned algorithm is developed further. The threshold values are adjusted only, if the estimated channel quality at the mobile station is within a certain band of the current threshold value.
Dynamical adjustment of the thresholds is discussed in “A Method for Outer Loop Rate Control in High Data Rate Wireless Networks”, by D. Paranchych and M. Yavuz, in IEEE VTC'2002 Spring, Vol. 3, pp. 1701-1705. This method is presented in connection with an IS-865 system, which is a CDMA cellular air interface. In IS-865, a data rate control mechanism is employed in the downlink (forward link) direction to adapt the transmitted data rate to changing channel conditions. An access terminal (a mobile station) measures the channel conditions, for example the received pilot Ec/Nt, for determining the maximum achievable data rate for the downlink transmission. Data transmission must meet a certain maximum average packet error rate (PER) requirement, which is defined for standard access terminals. The proposed method is to select the data rate using thresholds for the received pilot Ec/Nt, and to adjust the thresholds for received pilot Ec/Nt so that the desired PER is achieved. The method is thus a rate control method, where the received pilot Ec/Nt is used in an inner loop in the access terminal. The outer loop control, implemented also in the access terminal, monitors the received PER and adjusts the thresholds used in the inner loop so that a transmission rate achieving the desired PER is used. The following procedure is carried out for adjusting the thresholds. If a downlink packet is received in error, the thresholds are increased. If a downlink packet is received successfully, the thresholds are decreased. The threshold for the current transmission rate in increased/decreased with one step size, and the thresholds for all transmission rates are increased/decreased with a second step size.
Another technique is Hydrid Automatic Repeat reQuest (HARQ). The HARQ scheme combines forward error correction (FEC) and Automatic Repeat reQuest (ARQ). In HARQ, a receiver acknowledges a successfully received frame with an ACK message. Upon receipt of ACK, the transmitter knows that retransmissions are unnecessary. If a frame is received in error, the receiver sends a negative acknowledgement NAK to the transmitter and saves the received symbols. The transmitter consequently retransmits the frame. The receiver receives second symbols relating to the retransmitted frame, and soft-combines the stored first symbols and the received second symbols. This soft-combining reduces the error rate of the retransmitted frames, as previously received information relating to the same frame is employed.
Adaptive HARQ is a version of HARQ, where the data rate for each transmission and each retransmission may be adapted. In non-adaptive HARQ, the method for selecting the data rates in each retransmission is the same as that for selecting the data rates in each initial transmission.
It is proposed to use both fast closed loop rate control and adaptive HARQ in some communication systems for providing high-speed packet data transmission. The problem is that, for example, the outer loop rate control method proposed by D. Paranchych and M. Yavuz does not consider the effect of adaptive HARQ and cannot maintain the performance of each HARQ (re)transmission to a predetermined level. This means, for example, that too much transmission power is used in the retransmissions so that the power is wasted and the HARQ gain is not well utilized. Alternatively, too little transmission power may be used in the retransmission so that the residual FER is high and the HARQ gain is again not well utilized.
Dynamic thresholds for transmission rate selection are advantageous over the fixed thresholds. Adjusting the thresholds is crucial to the system throughput and residual Packet Error Rate (PER). Proper thresholds reduce the transmit power and keep the communication channel quality in a given level under different channel conditions. Adjusting the thresholds properly may thus increase the throughput of interference-limited wireless communications systems.
There is thus need for controlling transmission rate in combination with possible retransmissions.